package libnetwork import ( "fmt" "math/rand" "net" "strings" "sync" "time" log "github.com/Sirupsen/logrus" "github.com/docker/libnetwork/types" "github.com/miekg/dns" ) // Resolver represents the embedded DNS server in Docker. It operates // by listening on container's loopback interface for DNS queries. type Resolver interface { // Start starts the name server for the container Start() error // Stop stops the name server for the container. Stopped resolver // can be reused after running the SetupFunc again. Stop() // SetupFunc() provides the setup function that should be run // in the container's network namespace. SetupFunc() func() // NameServer() returns the IP of the DNS resolver for the // containers. NameServer() string // SetExtServers configures the external nameservers the resolver // should use to forward queries SetExtServers([]string) // FlushExtServers clears the cached UDP connections to external // nameservers FlushExtServers() // ResolverOptions returns resolv.conf options that should be set ResolverOptions() []string } const ( resolverIP = "127.0.0.11" dnsPort = "53" ptrIPv4domain = ".in-addr.arpa." ptrIPv6domain = ".ip6.arpa." respTTL = 600 maxExtDNS = 3 //max number of external servers to try extIOTimeout = 4 * time.Second defaultRespSize = 512 maxConcurrent = 100 logInterval = 2 * time.Second maxDNSID = 65536 ) type clientConn struct { dnsID uint16 respWriter dns.ResponseWriter } type extDNSEntry struct { ipStr string extConn net.Conn extOnce sync.Once } // resolver implements the Resolver interface type resolver struct { sb *sandbox extDNSList [maxExtDNS]extDNSEntry server *dns.Server conn *net.UDPConn tcpServer *dns.Server tcpListen *net.TCPListener err error count int32 tStamp time.Time queryLock sync.Mutex client map[uint16]clientConn } func init() { rand.Seed(time.Now().Unix()) } // NewResolver creates a new instance of the Resolver func NewResolver(sb *sandbox) Resolver { return &resolver{ sb: sb, err: fmt.Errorf("setup not done yet"), client: make(map[uint16]clientConn), } } func (r *resolver) SetupFunc() func() { return (func() { var err error // DNS operates primarily on UDP addr := &net.UDPAddr{ IP: net.ParseIP(resolverIP), } r.conn, err = net.ListenUDP("udp", addr) if err != nil { r.err = fmt.Errorf("error in opening name server socket %v", err) return } // Listen on a TCP as well tcpaddr := &net.TCPAddr{ IP: net.ParseIP(resolverIP), } r.tcpListen, err = net.ListenTCP("tcp", tcpaddr) if err != nil { r.err = fmt.Errorf("error in opening name TCP server socket %v", err) return } r.err = nil }) } func (r *resolver) Start() error { // make sure the resolver has been setup before starting if r.err != nil { return r.err } if err := r.setupIPTable(); err != nil { return fmt.Errorf("setting up IP table rules failed: %v", err) } s := &dns.Server{Handler: r, PacketConn: r.conn} r.server = s go func() { s.ActivateAndServe() }() tcpServer := &dns.Server{Handler: r, Listener: r.tcpListen} r.tcpServer = tcpServer go func() { tcpServer.ActivateAndServe() }() return nil } func (r *resolver) FlushExtServers() { for i := 0; i < maxExtDNS; i++ { if r.extDNSList[i].extConn != nil { r.extDNSList[i].extConn.Close() } r.extDNSList[i].extConn = nil r.extDNSList[i].extOnce = sync.Once{} } } func (r *resolver) Stop() { r.FlushExtServers() if r.server != nil { r.server.Shutdown() } if r.tcpServer != nil { r.tcpServer.Shutdown() } r.conn = nil r.tcpServer = nil r.err = fmt.Errorf("setup not done yet") r.tStamp = time.Time{} r.count = 0 r.queryLock = sync.Mutex{} } func (r *resolver) SetExtServers(dns []string) { l := len(dns) if l > maxExtDNS { l = maxExtDNS } for i := 0; i < l; i++ { r.extDNSList[i].ipStr = dns[i] } } func (r *resolver) NameServer() string { return resolverIP } func (r *resolver) ResolverOptions() []string { return []string{"ndots:0"} } func setCommonFlags(msg *dns.Msg) { msg.RecursionAvailable = true } func shuffleAddr(addr []net.IP) []net.IP { for i := len(addr) - 1; i > 0; i-- { r := rand.Intn(i + 1) addr[i], addr[r] = addr[r], addr[i] } return addr } func createRespMsg(query *dns.Msg) *dns.Msg { resp := new(dns.Msg) resp.SetReply(query) setCommonFlags(resp) return resp } func (r *resolver) handleIPQuery(name string, query *dns.Msg, ipType int) (*dns.Msg, error) { addr, ipv6Miss := r.sb.ResolveName(name, ipType) if addr == nil && ipv6Miss { // Send a reply without any Answer sections log.Debugf("Lookup name %s present without IPv6 address", name) resp := createRespMsg(query) return resp, nil } if addr == nil { return nil, nil } log.Debugf("Lookup for %s: IP %v", name, addr) resp := createRespMsg(query) if len(addr) > 1 { addr = shuffleAddr(addr) } if ipType == types.IPv4 { for _, ip := range addr { rr := new(dns.A) rr.Hdr = dns.RR_Header{Name: name, Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: respTTL} rr.A = ip resp.Answer = append(resp.Answer, rr) } } else { for _, ip := range addr { rr := new(dns.AAAA) rr.Hdr = dns.RR_Header{Name: name, Rrtype: dns.TypeAAAA, Class: dns.ClassINET, Ttl: respTTL} rr.AAAA = ip resp.Answer = append(resp.Answer, rr) } } return resp, nil } func (r *resolver) handlePTRQuery(ptr string, query *dns.Msg) (*dns.Msg, error) { parts := []string{} if strings.HasSuffix(ptr, ptrIPv4domain) { parts = strings.Split(ptr, ptrIPv4domain) } else if strings.HasSuffix(ptr, ptrIPv6domain) { parts = strings.Split(ptr, ptrIPv6domain) } else { return nil, fmt.Errorf("invalid PTR query, %v", ptr) } host := r.sb.ResolveIP(parts[0]) if len(host) == 0 { return nil, nil } log.Debugf("Lookup for IP %s: name %s", parts[0], host) fqdn := dns.Fqdn(host) resp := new(dns.Msg) resp.SetReply(query) setCommonFlags(resp) rr := new(dns.PTR) rr.Hdr = dns.RR_Header{Name: ptr, Rrtype: dns.TypePTR, Class: dns.ClassINET, Ttl: respTTL} rr.Ptr = fqdn resp.Answer = append(resp.Answer, rr) return resp, nil } func truncateResp(resp *dns.Msg, maxSize int, isTCP bool) { if !isTCP { resp.Truncated = true } // trim the Answer RRs one by one till the whole message fits // within the reply size for resp.Len() > maxSize { resp.Answer = resp.Answer[:len(resp.Answer)-1] } } func (r *resolver) ServeDNS(w dns.ResponseWriter, query *dns.Msg) { var ( extConn net.Conn resp *dns.Msg err error writer dns.ResponseWriter ) if query == nil || len(query.Question) == 0 { return } name := query.Question[0].Name if query.Question[0].Qtype == dns.TypeA { resp, err = r.handleIPQuery(name, query, types.IPv4) } else if query.Question[0].Qtype == dns.TypeAAAA { resp, err = r.handleIPQuery(name, query, types.IPv6) } else if query.Question[0].Qtype == dns.TypePTR { resp, err = r.handlePTRQuery(name, query) } if err != nil { log.Error(err) return } proto := w.LocalAddr().Network() maxSize := 0 if proto == "tcp" { maxSize = dns.MaxMsgSize - 1 } else if proto == "udp" { optRR := query.IsEdns0() if optRR != nil { maxSize = int(optRR.UDPSize()) } if maxSize < defaultRespSize { maxSize = defaultRespSize } } if resp != nil { if resp.Len() > maxSize { truncateResp(resp, maxSize, proto == "tcp") } writer = w } else { queryID := query.Id for i := 0; i < maxExtDNS; i++ { extDNS := &r.extDNSList[i] if extDNS.ipStr == "" { break } extConnect := func() { addr := fmt.Sprintf("%s:%d", extDNS.ipStr, 53) extConn, err = net.DialTimeout(proto, addr, extIOTimeout) } // For udp clients connection is persisted to reuse for further queries. // Accessing extDNS.extConn be a race here between go rouines. Hence the // connection setup is done in a Once block and fetch the extConn again extConn = extDNS.extConn if extConn == nil || proto == "tcp" { if proto == "udp" { extDNS.extOnce.Do(func() { r.sb.execFunc(extConnect) extDNS.extConn = extConn }) extConn = extDNS.extConn } else { r.sb.execFunc(extConnect) } if err != nil { log.Debugf("Connect failed, %s", err) continue } } // If two go routines are executing in parralel one will // block on the Once.Do and in case of error connecting // to the external server it will end up with a nil err // but extConn also being nil. if extConn == nil { continue } log.Debugf("Query %s[%d] from %s, forwarding to %s:%s", name, query.Question[0].Qtype, extConn.LocalAddr().String(), proto, extDNS.ipStr) // Timeout has to be set for every IO operation. extConn.SetDeadline(time.Now().Add(extIOTimeout)) co := &dns.Conn{Conn: extConn} // forwardQueryStart stores required context to mux multiple client queries over // one connection; and limits the number of outstanding concurrent queries. if r.forwardQueryStart(w, query, queryID) == false { old := r.tStamp r.tStamp = time.Now() if r.tStamp.Sub(old) > logInterval { log.Errorf("More than %v concurrent queries from %s", maxConcurrent, extConn.LocalAddr().String()) } continue } defer func() { if proto == "tcp" { co.Close() } }() err = co.WriteMsg(query) if err != nil { r.forwardQueryEnd(w, query) log.Debugf("Send to DNS server failed, %s", err) continue } resp, err = co.ReadMsg() if err != nil { r.forwardQueryEnd(w, query) log.Debugf("Read from DNS server failed, %s", err) continue } // Retrieves the context for the forwarded query and returns the client connection // to send the reply to writer = r.forwardQueryEnd(w, resp) if writer == nil { continue } resp.Compress = true break } if resp == nil || writer == nil { return } } if writer == nil { return } if err = writer.WriteMsg(resp); err != nil { log.Errorf("error writing resolver resp, %s", err) } } func (r *resolver) forwardQueryStart(w dns.ResponseWriter, msg *dns.Msg, queryID uint16) bool { proto := w.LocalAddr().Network() dnsID := uint16(rand.Intn(maxDNSID)) cc := clientConn{ dnsID: queryID, respWriter: w, } r.queryLock.Lock() defer r.queryLock.Unlock() if r.count == maxConcurrent { return false } r.count++ switch proto { case "tcp": break case "udp": for ok := true; ok == true; dnsID = uint16(rand.Intn(maxDNSID)) { _, ok = r.client[dnsID] } log.Debugf("client dns id %v, changed id %v", queryID, dnsID) r.client[dnsID] = cc msg.Id = dnsID default: log.Errorf("Invalid protocol..") return false } return true } func (r *resolver) forwardQueryEnd(w dns.ResponseWriter, msg *dns.Msg) dns.ResponseWriter { var ( cc clientConn ok bool ) proto := w.LocalAddr().Network() r.queryLock.Lock() defer r.queryLock.Unlock() if r.count == 0 { log.Errorf("Invalid concurrent query count") } else { r.count-- } switch proto { case "tcp": break case "udp": if cc, ok = r.client[msg.Id]; ok == false { log.Debugf("Can't retrieve client context for dns id %v", msg.Id) return nil } log.Debugf("dns msg id %v, client id %v", msg.Id, cc.dnsID) delete(r.client, msg.Id) msg.Id = cc.dnsID w = cc.respWriter default: log.Errorf("Invalid protocol") return nil } return w }